Transmission system



Aug. 9, 1938. H. 5. BLACK 2,126,529

TRANSMISSION SYSTEM.

' Filed Sept. 16, 1936 0.6. BY-HISS CIRCUIT INVENTOR h. S. BLA CK ATTORNEV Patented Aug. 9, 1938 UNITED STATES PATENT OFFICE TelephoneLaboratories,

Incorporated, New

York, N. Y.-, a corporation of New York Application September 16, 1936,Serial No. 101,058

6 Claims.

This invention relates to communication systems and has for its objectto improve transmission in such systems.

In transmission systems employing amplifiers,

the transmission is often controlled by means of an equalizer in theinput circuit which is in turn.

controlled by means of a pilot current which is taken from the output ofthe amplifier.

In such a variable equalizer as this, it is essen-' tial that the seriesand shunt resistances maintain a definite relation, so that the inputimpedance will remain nearly constant over the variable range of theequalizer characteristic and further keep the equalizer input impedancefrom being unduly sensitive to the termination used at the other end ofthe equalizer.

In accordance with the present invention an input equalizer iscontrolled through the medium of thermally sensitive elements, forexample, particularly elements of silver sulphide or boron so arrangedthat whenever the series resistance is changed the shunt resistance isautomatically changed to such a value that their product is heldconstant. I 25 The attached drawing shows a specific embodiment of theinvention in a pilot channel system of the general type shown in Patent1,918,390

granted to E. I. Green on July 18, 1933. The

amplifier may be of the type shown in Patent No.

2,075,975, granted to F. A. Brooks on April 6, 1937, while the equalizermay be of the type shown in Patent 1,606,817 granted November 16, 1926to G. H. Stevenson or more specifically of the type shown in Patent No.2,096,027, granted to H. W. Bode on October 19, 1937.

Current incoming over conductors I and 2 passes through equalizer 3having a series element 4 and a shunt element 5. It is then fed toamplifier li through shielded transformer and from the amplifier to theoutgoing conductors 8 and 9 through transformer l0.

The pilot frequency is picked up from the outgoing conductors 8 and 9 bymeans of the band pass filter H and fed through the full wave rectiiierH to relay l3.

In place of an adjustable resistance in the series element 4 of theequalizer, there is substituted a thermally sensitive resistance M.Similarly a thermally sensitive resistance I5 is substituted for anadjustable resistance in the shunt element 5 of equalizer 3.

Resistance I4 is enclosed in an oven 20 in which is also included asecond similar thermally sensitive resistance It and a heating elementII which enwraps both resistances l4 and I6. Resistance 5 is likewiseassociated with a second similar thermally sensitive resistance I! and aheating element l9, all enclosed in an oven 2|.

Thermal resistances l6 and N form opposite arms of a Wheatstone bridgeof which resistances 22 and 23, which are unaffected by temperature,form the alternate arms. The bridge receives current from thealternating current generator 26 through transformer 21 and the diagonalof the bridge includes full wave rectifier 2t and relay 25. Relay l3controls heater l8 and relay 25 controls heater l9.

According to the well-known Wheatstone bridge formula when the bridge isbalanced Since resistances M and I5 are similar to resistances 6 and II,respectively, and are subjected to the same heat conditions, the productR14 R15 is also a constant when the bridge is balanced. r

Since silver sulphide has a high negative tem- -perature-resistancecoeflicient, its resistance decreases rapidly with increase intemperature. Therefore, when the pilot frequency is below the desiredvalue, relay I3 is released, the heating circuit of oven 20 is closedand the resistance of the elements I4 and I6 decreases, the decrease inresistance of element l4 causing the amplification to increase, whichwill be reflected by an increased volume for thepilot frequency untilthe desired point is reached and relay l3 operates to turn off the heat.ance of element I6 unbalances the bridge, operating relay 25, whichopens the heating circuit of oven 2|, causing the temperature of thatoven to fall and the resistance of elements l5 and H to increase untilthe balance is restored and the aforementioned. relation of theresistances adjusted.

There are three conditions which may exist with regard to resistance 4and therefore with regard to resistance l6. Namely, it may be constant,it may be increasing or it maybe decreasing. If resistance M is constantand oven 2| is too hot, resistance I5 will be too low, the bridge willbe unbalanced andrelay 25 will operate, turning ofi the heat, coolingthe oven 2|, increasing the resistance I5 and bringing the bridge toward a balance. As the balance is approached relay 25 releases, heat isagain supplied to oven ill The reduced resist- 2|, producing a too hotcondition, which again turns of! the heat and this cycle will continueas long as oven 20 is at a constant temperature.

Therefore, there are two requirements that must be met to make thesystem operative in this condition, namely: the heating current for oven2i must be great enough to insure this oscillatory condition justdescribed; and when heat is applied to oven 2| the resistance I5 must beable to assume a resistance value somewhat less than is required tobalance the bridge.

When oven 20 is heating so that resistance I4 is decreasing, to balancethe bridge, resistance l5 must increase and oven 2| must cool. Sinceoven 2| is normally too hot during operation, the decrease in resistancel4 lengthens the time that relay 25 remains operated to hold the heatingcircuit for oven 2! open.

When oven 20 is cooling so that resistance I4 is increasing, resistanceI5 must decrease and therefore oven 2L must heat. In order for this tobe true, oven 2| must be able to increase in temperature faster thanoven 20 can decrease in temperature, which would naturally be the case.During the time that oven 20' is cooling, the oscillatory condition ofoven 2! previously described continues, with the released periods ofrelay 25 increased in relative length in order to hold the temperatureat a higher point.

This arrangement therefore provides a simple,

stable means of maintaining the output of the amplifier constant whilepreserving the proper family of characteristics for equalizer 3 and atthe same time preserving a good impedance looking into equalizer 3. Forthe specific embodiment described the reflection coefflcient at theinput to equalizer 3 referred to the resistance of the equalizer isbetter than that of the equalizers termination, the improvement beingequal to the amplification or current ratio corresponding to twice theinsertion loss of the equalizer. For example, if the equalizer loss be 3db., this improvement would be fourfold or in other words the refiectioncoeflicient at the input would be that of the termination.

What is claimed -is:

1. In a transmission system, an amplifier, an equalizer in the input ofsaid amplifier having a series resistance and a shunt resistance, andmeans for maintaining the output of said amplifier constant, comprisingmeans under the control of said output to vary said series and shuntresistances, while maintaining their product constant, said varyingmeans including silver sulphide resistance units, and heating means forreciprocally varying the temperatures thereof.

2. In a transmission system, an amplifier, an equalizer in the input ofsaid amplifier, having a series resistance element and a shuntresistance element, and means for maintaining the output of saidamplifier constant, comprising means under the control of said output tovary the value of said series and shunt resistance elements, whilemaintaining their product constant, saidvarying means comprisingauxiliary resistance units of large temperature coeflicients ofresistance in the opposite arms of a Wheatstone bridge.

3. In a transmission system, an amplifier, an

equalizer in the input of said amplifier having a series resistance anda shunt resistance, means for maintaining the output of said amplifierconstant, comprising means under the control of said output to vary theseries and shunt resistances of said equalizer, while maintaining theirproduct constant, said varying means comprising silver sulphideresistance units in the opposite arms of a Wheatstone bridge, and meanscontrolled by said Wheatstone bridge to control the relativetemperatures of said silver sulphide resistance units.

4. In a transmission system, an amplifier, an equalizer in the input ofsaid amplifier, said equalizer having a series element and a shuntelement, and means for maintaining the output of said amplifier constantwhile preserving the proper characteristics for said equalizercomprising a temperature sensitive resistance in the series element ofsaid equalizer, a temperature sensitive resistance in the shunt elementof said equalizer, means under the control of the output of saidamplifier to vary the temperature of said series temperature-sensitiveresistance, a Wheatstone bridge associated with said series and shunttemperature sensitive resistances and means under the control of saidbridge 'to vary the temperature of said shunt temperature sensitiveresistance.

5. In a transmission system, an amplifier, an equalizer in the input ofsaid amplifier, said equalizer having a series element and a shuntelement, and means for maintaining the output of said amplifier constantwhile preserving the proper characteristics for said equalizercomprising a silver sulphide resistance in the series element. of saidequalizer, a silver sulphide resistance in the shunt element of saidequalizer, means under the control of the output of said amplifier tovary the temperature of said series silver sulphide resistance, aWheatstone bridge associated with said series and shunt silver sulphideresistances and means under the control of said bridge to reciprocallyvary the temperature of said shunt silver sulphide resistance.

6. In a transmission system, an amplifier, an equalizer in the input ofsaid amplifier, said equalizer having a series element and a shuntelement, and means for maintaining the output of said amplifier constantwhile preserving the proper characteristics for said equalizercomprising a silver sulphide resistance in the series element of saidequalizer, a silver sulphide resistance in the shunt element of saidequalizer, said series silver sulphide resistance enclosed in an oventogether with a first auxiliary silver sulphide resistance and a heater,said shunt silver sulphide resistance enclosed in a second oven togetherwith asecond auxiliary silver sulphide resistance and a second heater,means under the control of the output of said amplifier to vary thetemperature of said first oven, at Wheatstone bridge including saidfirst and second auxiliary silver sulphide

